Magnetic memory cell

ABSTRACT

A magnetic memory device based on easy domain wall propagation and the extraordinary Hall effect includes a perpendicular-to-plane a magnetic electrically conductive element ( 2 ) that includes a memory node ( 3 ). Electrical conductors ( 12 - 15 ) surround the node ( 3 ) so that when energised, a magnetic field is produced to change the magnetisation state of the node ( 3 ). In memory state “0” a magnetic domain is pinned within tapered portion ( 5 ) of the element ( 2 ). When a magnetic field is applied to the device, the domain (D) becomes unpinned and extends into the node ( 3 ) to produce a “1” state. The state of magnetisation is sensed by means of a Hall contact ( 11 ). The current pulse (J c ) is applied through the element ( 2 ) so that the Hall voltage can be detected.

FIELD OF THE INVENTION

[0001] This invention relates to a magnetic memory cell that hasparticular but not exclusive application to memory devices which includea memory array.

BACKGROUND

[0002] Hitherto, non-volatile random access memories (MVRAMs) have beenproposed including magnetic random access memories (MRAMs) andferroelectric RAMs (FRAMs), as candidates to replace conventionaldynamic random access memories (DRAMs) and hard disc drives. A number ofdevices have been proposed which make use of giant magneto resistance(GMR) in multi-layers, as described in M. N. Baibich, J. M. Broto, A.Fert, F. Nguyen van Dau, F. Petroff, P. Erienne, G. Gruzet, A.Friedrich, and J. Chazelas, Phys. Rev. Lett 61, 2472 (1988). Also,devices based on tunnelling magneto resistance (TMR) in magnetic tunneljunctions have been proposed as described in J. F. Bobo, F. B. Mancoff,K. Bessho, M. Sharma, K. Sin, D. Guarisco, S. X. Wang and B. M. Clemens,J. Appl. Phys. 83 6685 (1998). In these devices, information is storedin terms of the orientation of the magnetisation of small patternedstructures and is read by measuring resistance. Previously proposed MRAMdevices utilise in-plane magnetised magnetic layers together with a GMRor TMR measurement.

[0003] Another MRAM device has been proposed that utilises Hall effectmeasurements in a essentially two dimensional electron gas semiconductormulti-layer as described in F. G. Monzon, M. Johnson and M. L. Roukes,Appl. Phys. Lett. 71, 3087 (1997).

[0004] The use of in-plane magnetised magnetic layers imposes arestriction on the amount of downscaling that can be achieved in orderto miniaturise the devices. Downscaling is limited by the increasingeffect of the demagnetising field, generated by the magnetic sinks andsources of the magnetisation at the boundaries of the device. Thiscauses a decrease in the effective magnetic anisotropy as the cell sizeis decreased, until the super-parametric limit is reached where the cellmagnetisation becomes unstable at room temperature.

SUMMARY OF THE INVENTION

[0005] With a view to overcoming this problem, the present inventionprovides a magnetic memory cell comprising: an elongate, magneticconductive element, a magnetic field producing device to apply amagnetic field to the conductive element, and a contact to allowdetection of a Hall voltage developed across the element, the magneticconductive element being configured to allow a magnetic domain to beinduced therein and to provide pinning for its domain wall whereby thedomain adopts a first configuration or a second different pinnedconfiguration dependant upon the magnetic field produced by the magneticfield producing device, and such that first and second different valuesof the Hall voltage are produced for said respective domain wallconfigurations when an electrical current is passed through the magneticconductive element.

[0006] The magnetic conductive element may comprise at least one planarlayer of magnetic material with its easy or preferred axis ofmagnetisation extending transversely to the plane thereof, moreparticularly perpendicular to the plane. In this way, the influence ofthe demagnetising field is significantly less dependent on the lateralcell size than hitherto, which facilitates downscaling of the cell forpurposes of miniaturisation. Layers of the magnetic material may beconfigured between layers of non-magnetic electrically conductivematerial. One material system may comprise layers of cobalt andplatinum, which may be formed as a superlattice. Alternatively,τ-Mn_(0.6-x)Ni_(x)Al_(0.4) where x<0.08 may be used.

[0007] The magnetic conductive element may include a region of reducedcoercivity to promote formation of the induced magnetic domain. Theregion of reduced coercivity may be produced by focussed ion beamirradiation, for example using He⁺ ions at high fluency or Ga⁺ ions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In order that the invention may be more fully understood anembodiment thereof will now be described by way of example withreference to the accompanying drawings in which.

[0009]FIG. 1 is a schematic sectional view of a memory cell inaccordance with the invention taken along the line A-A′ of FIG. 2;

[0010]FIG. 2 is a plan view of a memory cell in accordance with theinvention;

[0011]FIG. 3 illustrates the perpendicular component of the magneticfield produced along the line A-A′ by energisation of field producingconductors shown in FIG. 2;

[0012]FIG. 4 illustrates a memory cell before initialisation;

[0013]FIG. 5 is a schematic illustration of the memory cell in its “0”state;

[0014]FIG. 6 illustrates a memory cell in its “1” state;

[0015]FIG. 7a is a graph illustrating magnetic field pulses applied tothe device;

[0016]FIG. 7b illustrates the extraordinary Hall effect voltage producedin response to the applied magnetic pulses shown in FIG. 7a;

[0017]FIGS. 8a-8 e illustrate process steps for fabrication of thedevice; and

[0018]FIG. 9 illustrates a memory cell array in accordance with theinvention.

DETAILED DESCRIPTION

[0019] Overview of Device

[0020] Referring to FIGS. 1 and 2, a memory cell C according to theinvention is fabricated on a (0001) oriented sapphire substrate 1 whichis overlaid by an elongate magnetic, electrically conductive element orstripe 2 which, as shown in FIG. 2 includes a central, ovoid orgenerally circular portion 3 which acts as a memory node and portions 4,5 which have tapered edges 6, 6′ so that edge discontinuities 7, 7′ areprovided between each of the tapered portions 4, 5 and the central,circular portion 3.

[0021] The magnetic, conductive strip 2 comprises a thin (˜5 nm)intermediate buffer layer 8 of (0001) orientated platinum (Pt) with awidth w less than 1 μm, which is overlaid by alternating layers 9, 10 ofcobalt (Co) and Pt. The thickness of the layers 9, 10 is less 2 nm andmay be deposited by sputtering. Thus, a layer system of non-magnetic,electrically conductive Pt layers and magnetic, conducting Co layers isproduced, containing edge discontinuities 7, 7′.

[0022] A Hall contact 11 extends transversely in the −x direction fromthe node 3. The contact 11 is fabricated of a conductive material suchas Pt and used to detect an extraordinary Hall effect (EHE) voltage aswill be explained in more detail hereinafter. A layer of insulatingmaterial, for example SiO₂, overlies the current stripe 2 and the Hallcontact 11.

[0023] The node 3 is surrounded by a magnetic field producing device inthe form of transverse electrically conductive stripes 12-15 disposed ina cruciform arrangement which generates a magnetic field componentH_(perp) perpendicular to the x-y plane when a current is passed throughthe stripes in the direction of arrows 16. It will be seen that thecombined effect of the currents flowing in the stripes 12-15, gives riseto a magnetic field shown schematically in FIG. 3. Thus, around the node3, the conductors act the manner of Helholtz coil to produce aperpendicular field component that is directed normally to the plane ofsubstrate 1. In the coordinate scheme shown in FIGS. 1 and 2, thestripes 12-15 lie in the x-y plane, and the perpendicular magneticcomponent field vector extends in the z direction.

[0024] As can be seen from FIG. 3, the field strength within the areaenclosed by the cruciform intersection of the stripes 12-15 is greaterthan outside the enclosed area and so the stripe arrangement allows amagnetic field to be applied to the node 3 selectively. The stripes 12,13 are formed beneath the node 3 and the Hall contact 11 within thesubstrate 1 and may typically be formed of aluminium. The stripes 14, 15overlie the insulating layer 12, above the node 3 and may be made ofaluminium.

[0025] A region 17 in tapered portion 5 of stripe 2 has much lowercoercivity than the remainder of the stripe. This low coercivity regionis produced by ion irradiation as will be explained hereinafter.

[0026] Writing Data

[0027] In order to store data, a current is applied to the magneticfield producing stripes 12-15 in the direction of arrows 16 so as tochange the magnetisation of the conductive stripe 2. When a current ispassed through the stripe 2, an EHE voltage is produced in Hall contact11, dependent upon the magnetic state of the stripe 2. The level of EHEvoltage is dependent upon the configuration of a magnetic domain inducedin the stripe 3 as a result of the applied magnetic field. This will nowbe explained in more detail with reference to FIGS. 4 to 7. The memorycell is initialised by applying a strong negative magnetic field pulsein order to saturate the magnetisation in the stripe 2 in the −zdirection i.e. perpendicular to the x-y plane containing the magneticstripe 2, as shown by pulse 18 in FIG. 7a. Thereafter, a weak positivemagnetic field i.e. in the +z direction is applied reference 19 in FIG.8a. This weak, positive magnetic field promotes nucleation of a magneticdomain in the region 17 shown in FIG. 4. The magnetisation of the domainis reversed compared to the remainder of the initialised stripe 2. Thedomain preferentially nucleates within the region of reduced coercivity17. The resulting domain D is delimited by a domain wall DW that is justa few nm thick and behaves rather like a soap bubble. It always attemptsto minimise its surface (domain wall length) by extending outwardly fromits nucleation centre in region 17, until it becomes pinned, as shown inFIG. 5, the pinning being produced by the edge discontinuities 7. Thiscorresponds to a “0” stored memory state.

[0028] In order to write a “1” state onto the memory node, a stronger,positive (+z) field pulse is applied, referenced 20 in FIG. 7a. Thestronger magnetic field overcomes the effect of the pinning for the “0”state and as a result, the domain wall DW expands past the pinningproduced by the discontinuities 7, fills the circular node region 3 andis pinned by the discontinuities 7′.

[0029] The “0” state can be re-written by applying a negative fieldpulse of preferably equal strength to that used to write “1” state,namely pulse 21 shown in FIG. 7a. The negative going (−z) magnetic fieldresults in a contraction of the domain D and its domain wall DW so thatthe domain D again becomes pinned by the discontinuities 7 as shown inFIG. 5. It will be seen from a comparison of FIGS. 5 and 6 that themagnetisation of the Pt/Co layers in the memory node 3 is reversedbetween a “0” state shown in FIG. 5 and a “1” state shown in FIG. 6 as aresult of the magnetic domain either being pinned by the edgediscontinuities 7 or 7′. The resulting magnetisation of the node 3 isthus switched between opposite senses for the “0” and the “1” stateswith the resulting magnetic field vector extending perpendicularly tothe plane of the stripe 2.

[0030] The state of the node 3 can be read by measuring a Hall voltageon contact 11. A current J_(c) is applied to the stripe 2. The currentmay comprise current pulses with a current density of ˜10⁷ A/cm², d.c.˜0.5v, f˜500 MHz or simply a single current pulse with a current densityof ˜10⁷ A/cm², τ˜1 ms. The magnetisation state of the node 3 results ina Hall voltage being developed on contact 11 in response to the currentpulse, the Hall voltage being dependent upon the magnetisation state ofnode 3. Thus, for the “0” state shown in FIG. 5, a relatively low Hallvoltage is produced whereas for the “1” state shown in FIG. 6, arelatively high Hall voltage is produced. With the relatively smalldimensions and field strengths described herein, the extraordinary Halleffect dominates and conventional Hall resistivity is not a predominantfactor. The detectable Hall signal of the cell is approximately 75% ofVH=2IpH/t, where I is the applied current, t is the film thickness andpH. is the (extraordinary) Hall resistivity, a temperature-dependentmaterial constant. The factor 2 is due to the change of magnetisationorientation from anti-parallel to parallel direction with respect to theapplied field. The current-density j=l(t,d_(c)) should be arranged sothat the sample will not be thermally destroyed during operation, andtypically should not go beyond ˜5×10⁷ A/cm² for pulsed currents.

[0031] Typical values of pH in Pt/Co/Pt layer configurations and Co/Ptsuperlattices is approximately 0.6 μΩcm as described in J. Caulet, C.Train, V. Mathet, R. Laval, B. Bartenlian, P. Veillet, K. Le Dang, C.Chappert and C. L. Canedy, X. W. Li and Gang Xiao, Phys. Rev. B62 508(2000). This corresponds to a maximal signal change between the “1” and“0” states of 3 mv at the Hall contact 11.

[0032]FIG. 7b illustrates the EHE voltage developed at contact 11 inresponse to the sequence of magnetic field pulses 18-21 shown in FIG. 7aAs regards the temperature range of operation, both the domain wallvelocity and the extraordinary Hall resistivity increase with increasingoperating temperature of the device. These tendencies improve theoperational characteristics of the device, by improving the switchingspeed between the “0” and “1” states and increasing the signal level.However, the pinning strength of the domain wall, which corresponds tothe stability of the written data, decreases with increasingtemperature. The working temperature of the device is close to roomtemperature and can be optimised by an appropriate selection of themagnetic layer system used for the memory node.

[0033] Device Fabrication

[0034] Fabrication of the device will now be described with reference toFIG. 8. Referring to FIG. 8a, the sapphire substrate 1 is subject toe-beam lithography and etching in order to provide trenches 22, 23 and24. The trenches 22 and 23 correspond to the location of the lowermostconductive stripes 12, 13 shown in FIG. 1. Trench 24 corresponds to thelocation of a conductor 25 to be connected to the Hall contact 11.Thereafter, suitable electrical conductive material is sputtered intothe trenches 22, 23 and 24, for example aluminium. The upper surface 26of the substrate 1 is then polished by a suitable conventional techniqueto provide the configuration shown in FIG. 8a. Then as illustrated inFIG. 8b, the Pt buffer layer 8 and the Co—Pt layers 9, 10 are depositedby sputtering. Pt (111) can grow epitaxially on a sapphire Al₂O₃(0001)surface. Reference is directed to R. Farrow, G. R. Harp, R. F. Marks, T.A. Rabedeau, M. F. Toney, D. Weller, S. S. P. Parkin, J. Cryst. Growth122, 47 (1993). In order to prepare the surface 26 for the sputteringprocess, chemical surface reconstruction is carried out prior todeposition of the layers 8, 9, 10, by dipping the substrate in asolution of NH₄OH (28%): H₂O₂ (30%): H₂O (1:1:100), for two minutes.After rinsing with ethanol and water, the substrate is dried in nitrogengas and the introduced into a sputtering chamber (not shown) where it isannealed for 20 minutes at a temperature of 650° C.

[0035] Two different sputtering techniques are employed to create thePt—Co films on the substrate. In order to deposit the first, Pt bufferlayer 8, direct current (DC) sputtering is employed whereas radiofrequency (RF) sputtering is utilised to deposit the subsequent Co andPt layers 9,10.

[0036] For the deposition of the Pt layers 10, a magnetron cathode isused as the sputtering target and the resulting plasma is confined bymagnetic fields near the target, enhancing the deposition efficiency andoptimising the film growth. For the deposition of the Co films 9, sinceit is a ferromagnetic material, the target concentrates the magneticflux and no magnetron effect is utilised. Co is thus sputtered using thestandard RF diode technique.

[0037] The thin film deposition of the layers is carried out in foursuccessive steps. Initially, at an argon pressure of 5×10⁻³ mb and asubstrate temperature of 610° C., the buffer layer 8 is grownepitaxially on the sapphire surface using DC magnetron sputtering at agrowth rate of 2.5 Å/s, to a thickness of 40 Å. By such a technique, themismatch between the Pt (111) and Al₂O₃ (0001) is less than 1%. Theresulting buffer layer 8 thus comprises a (111) textured polycrystallinelayer, which is flat and continuous, with a surface roughness of 2-3 Åwhen measured with an atomic force microscope.

[0038] Next, a 5 Å Pt layer is sputtered in the RF mode at a rate of 0.2Å/s at a substrate temperature of 300° C. in order to optimise theinterface quality with the Co layer to be grown thereafter. Immediatelythereafter, Co and Pt layers 9, 10 are successively deposited by RFsputtering, maintaining the same sputtering conditions, namely asubstrate temperature of 300° C. and a growth rate of 0.2 Å/s. Theprocess is repeated so as to build up successive pairs of layers 9, 10.A final Pt cap layer of a thickness of the order of 3 nm, overlays theCo—Pt layer system. The resulting structure may comprise a [Pt(14Å)—Co(3 Å)]_(m<10)—Pt(14 Å) multi-layer.

[0039] By the use of RF sputtering with a low sputter growing rate ofthe order of 0.2 Å/s, high quality films are produced withoutsignificant intermixing of Co and Pt, as described in J. Wunderlich,Ph.D. Thesis, Institut d'Electronique Fondamentale (UniversitéParis-Sud, Orsay-France) and Max Planck Institut für Mikrostrukturphysik(Halle-Germany), ISBN 3-8265-9110-0, (2001). Then, high-resolutionelectron-beam lithography is employed to shape the sputtered layers intothe elongate stripe 2. The sputtering is carried out such that inregions 2 a, 2 b (FIG. 2) of the stripe each comprise solely a layer ofPt and the Co—Pt layer system is configured solely in the regions 3, 4,5.

[0040] A shown in FIG. 8b, a region 26 of insulating material such asSiO₂ is deposited by conventional sputtering and electron beamlithographic techniques, between the stripe 2 and the conductor 25.

[0041] Then as shown in FIG. 8c, the Hall contact 11 is formed bysputtering Pt according to the techniques just described, so as to forman electrical connection with the stripe 2 and also the buried conductor25. Thereafter, further insulating material 12, typically S_(i)O₂ issputtered over the resulting structure, as shown in FIG. 8d. Then, theupper conductive stripes 14, 15 are deposited using conventionallithographic and sputtering techniques so as to achieve the structureshown in FIG. 8e, which corresponds to FIG. 1.

[0042] Focussed ion beam lithography using He⁺ or Ga⁺ ions is used tocreate the reduced coercivity region 17, so as to provide an artificialnucleation centre for the magnetic domain D within the device. Theirradiation modifies the magnetic properties of the region 17 byinducing mixing at the Co/Pt interfaces, as described in C. Chappert etal, Science 280, 1919 (1998) and T. Aign, P. Meyer, S. Lemerie, J.-P.Jamet, J. Ferré, V. Mathet, C. Chappert, J. Gierak, C. Vieu, F.Rousseaux, H. Launois and H. Bernas, Phys. Rev. Lett. 81, 5656 (1998).The coercive field as well as the Curie temperature are thus decreasedin comparison with the non-irradiated regions as described in C. Vieu,J. Guerack, H. Launois, T. Aign, P. Meyer, J.-P. Jamet, J. Ferré, C.Chappert, V. Mathet, H. Bernas, Microelectronic Engineering 53, 191(2000). Whilst the region 17 is shown in the tapered portion 5 of stripe2, it will be understood that it could be located elsewhere, for examplein the tapered portion 4.

[0043] From the foregoing, it will be understood that the memory cell inaccordance with the invention stores information in terms of theconfiguration of a single magnetic domain wall DW in a nano-structuredmedia, in particular a Pt/Co/Pt layer system in which a magnetisationreversal process can be carried out. Detailed studies of this reversalprocess in thin Pt/Co films and similar thin layer systems that have aneasy magnetisation axis perpendicular to the film plane are described inS. Lemerle, PhD. Thesis, Université Paris XI, Orsay (1998) and J.-P.Jamet, S. Lemerle, P. Meyer and J. Ferré, Phys. Rev. B57, 14319 (1998).The magnetisation reversal starts by nucleation of a domain of oppositemagnetisation, delimited by the domain wall, and continues by the growthof the magnetic domain due to domain wall propagation, as described withreference to FIGS. 5 and 6.

[0044] As previously explained, it has been found that the magneticdomain D behaves like a two dimensional soap bubble and always attemptsto minimise its surface area and thus can be subjected to pinning. Thepinning can be due to the soap bubble like behaviour of the domain whereit contacts the discontinuities 7, 7′ shown in FIG. 2 and also may bedue to inhomogenities of the demagnetisation energy density which occurin the region of the sharp edges 7, 7′. This is discussed in more detailin J. Wunderlich, Ph.D. Thesis, Institut d'Electronique Fondamentale(Université Paris-Sud, Orsay-France) and Max Planck Institut fürMikrostrukturphysik (Halle-Germany), ISBN 3-8265-9110-0, (2001).

[0045] As previously explained, the different configurations of thedomain D for the “0” and “1” states shown in FIGS. 5 and 6, producedifferent levels of Hall voltage on contact 11. In the described device,the extraordinary Hall effect dominates. Two mechanisms that give riseto EHE have been identified, namely skew scattering and side jumpscattering. Skew scattering is described in J. Smit, Physica 24, 39(1958). Side jump scattering is described in L. Berger, Phys. Rev. B2,4559 (1970). These two mechanisms result from spin dependentasymmetrical scattering of spin-polarised electrons in the presence ofspin orbit coupling. Since the magnetism of transition metals such as Cois due to the spin polarisation of its d-electrons, the averagedorientation of the spin polarisation direction of these itinerantconduction electrons determines the macroscopic magnetisation. Since thein-plane current in a two-dimensional magnetic layer system is spinpolarised with respect to the perpendicular-to-plane component, the EHEdepends on the actual magnetisation distribution of theperpendicular-to-plane magnetised layer system.

[0046] It has been found that materials with a high spin-orbitinteraction such as Pt and Au demonstrate a high extraordinary Hallsignal and experiments have shown that an increase of Hall resistivityoccurs with temperature due to increasing phonon scattering. Thus by anappropriate selection of materials and conditions, the EHE resistivitycan be optimised in the device according to the invention.

[0047] Memory Cell Array

[0048] Referring now to FIG. 9, an example of a memory cell arrayaccording to the invention is shown that comprises a 2×2 array of memorycells C, although it will be appreciated that the principles of thedevice can be extended to much larger cell arrays. Cell C₁₁ correspondsto cell C of FIG. 2 and all the cells in the array have an identicalstructure. Each memory cell C is provided with its own magnetic fieldproducing device defined by upper and lower conductive stripes 14, 15,28 and 12, 13, 27, which are energised by current supply devices 29, 30,31 and 32. Thus, by appropriately energising the stripes, a magneticfield can be applied to the memory cells individually to enable them tobe selectively switched between the “0” and “1” memory states.

[0049] The magnetic, conductive stripes 2 for the cells concerned areconnected together in columns and the Hall contacts 11 of the cells areconnected together in rows by means of conductive stripes 34, 35 thatare buried in the substrate.

[0050] In order to read the memory state of a particular cell, a currentpulse supply circuit 36 applies a current pulse to the stripe 2 of thecell concerned For example when the memory cell C₁₁ is to be read, acurrent pulse is applied to line 37 so as to pass a current throughstripe 2 of cell C₁₁. A corresponding EHE voltage is produced on contact11 and is supplied through stripe 34 to a voltage sensor 38 connected toburied stripes 34, 35. Thus, the voltage received at sensor 38corresponds to the memory state i.e. “1” or “0”.

[0051] Many modifications and variations of the invention are possible.For example, whilst the described example makes use of a Co/Pt layersystem in the form of a superlattice at the memory node 3, it would bepossible to use a single ultra thin Co layer sandwiched between twoplatinum layers. In addition, the magnetic layer system may be modifiedby He⁺ ion irradiation, which induces intermixing at the interfacesbetween the Pt and Co. this decreases the coercivity of the magneticlayer system and promotes easy domain wall propagation at very lowapplied magnetic fields, of less than 100 Oe, as described in T.Devolder, J. Ferre, C. Chappert, H. Bernas, J-P jamet and V. Mathet,Phys. Rev. B 64,064415 (2001). The ion irradiation may increase the EHEdemonstrated by the device. Furthermore, other material systems can beused. For example, instead of using a sapphire substrate, the Pt—Colayer system can be grown on a S_(i)O₂ surface. Also,τ-Mn_(0.6-x)Ni_(x)Al_(0.4) where x<0.08 thin films epitaxially grown onGaAs can be utilised. Such films exhibit a large Hall resistivity of upto 7 μΩcm and demonstrate very square hysteresis loops at low appliedfields. This facilitates propagation of the domain wall at low fieldstrengths typically between 100-200Oe. More particularly, thin (˜10 nm)epitaxial films of τ-Mn_(0.6-x)Ni_(x)Al_(0.4) where x<0.08 alloys on a(001) oriented GaAs substrate may be used since they demonstrate strongEHE and easy domain wall propagation at relatively low magnetic fields(˜2KOe) as described by T. Sands et al, J. Appl. Phys. 73 (10), 6399(1993).

1. A magnetic memory cell comprising: an elongate, magnetic,electrically conductive element, a magnetic field producing device toapply a magnetic field to the conductive element, and a contact to allowdetection of a Hall voltage developed across the element, the magneticconductive element being configured to allow a magnetic domain to beinduced therein and to provide pinning for its domain wall whereby thedomain adopts a first configuration or a second different pinnedconfiguration dependant upon the magnetic field produced by the magneticfield producing device, and such that first and second different valuesof the Hall voltage are produced for said respective domain wallconfigurations when an electrical current is passed through the magneticconductive element.
 2. A memory cell according to claim 1 wherein themagnetic conductive element comprises at least one generally planarlayer of magnetic material with its easy axis of magnetisation extendingtransversely to the plane thereof.
 3. A memory cell according to claim 2wherein the or each of the layers of magnetic material are disposedbetween layers of non-magnetic electrically conductive material.
 4. Amemory cell according to claim 2 or wherein the magnetic conductiveelement comprises a sandwich of layers of Cobalt between layers ofPlatinum.
 5. A memory cell according to claim 2 wherein the conductiveelement comprises layers of τ-Mn_(0.6-x)Ni_(x)Al_(0.4) where x<0.08. 6.A memory cell according to claim 1 wherein the magnetic conductiveelement has a region of reduced coercivity to receive the inducedmagnetic domain.
 7. A memory cell according to claim 6 wherein theregion of reduced coercivity has been produced by ion irradiation.
 8. Amemory cell according to claim 1 wherein magnetic conductive elementincludes an edge discontinuity to provide said pinning of the domainwall.
 9. A memory cell according to claim 8 wherein the magneticconductive element comprises a portion with tapered edges and acontiguous portion with a generally circular periphery, such that theedge discontinuity occurs between the portions.
 10. A memory cellaccording to claim 9 wherein the tapered portion has been subject tolocalised ion irradiation to promote formation of the magnetic domaintherein.
 11. A memory cell according to claim 1 wherein the magneticconductive element has been ion irradiated.
 12. A memory cell accordingto claim 1, wherein the magnetic conductive element is configured on asubstrate, and the magnetic field producing device comprises conductivestrips overlying the substrate around the element.
 13. A memory cellaccording to claim 1 including a pulse generator to apply a currentpulse to the element to produce the Hall voltage.
 14. A memory deviceincluding an array of memory cells each as claimed in claim
 1. 15. Amethod of fabricating a memory device, comprising forming an elongate,magnetic, electrically conductive element on a substrate, providing amagnetic field producing device on the substrate for applying a magneticfield to the conductive element, and fabricating a contact to allowdetection of a Hall voltage developed across the element, andconfiguring the magnetic conductive element to allow a magnetic domainto be induced therein and to provide pinning for its domain wall so thatthe domain adopts a first configuration or a second different pinnedconfiguration dependant upon the magnetic field produced by the magneticfield producing device, and such that first and second different valuesof the Hall voltage are produced for said respective domain wallconfigurations when an electrical current is passed through the magneticconductive element.
 16. A memory device comprising a generally planarmagnetic and electrically conductive memory node with its easy axis ofmagnetisation extending generally perpendicular to said plane, and anelectrode to detect an extraordinary Hall effect voltage as a functionof the magnetisation state of the memory node.